1. Field of the Invention
The invention relates to an optical system of a microlithographic projection exposure apparatus, and to a microlithographic exposure method.
2. State of the Art
Microlithography is used to produce microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, which has an illumination device and a projection lens. The image of a mask (=reticle) illuminated via the illumination device is in this case projected via the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photo-resist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In the illumination device, various approaches are known for setting specific polarization distributions in the pupil plane and/or in the reticle in a targeted manner in order to optimize the image contrast. Here, the so-called IPS value (“Intensity in Preferred State”), which describes the degree of polarization in a desired state, is of fundamental importance. By way of example, an undesired reduction of the IPS value can in practice result from stress birefringence occurring in the optical elements or lens elements of the illumination device and this can more particularly lead to the polarization state becoming elliptical or, although still having the desired preferred polarization direction, having a light component that is not polarized in the desired direction. In this case, the IPS value can be increased by compensating for this ellipticity.
Proceeding from already polarized light (e.g. as a result of using a laser-light source supplying already polarized light), various approaches are known for achieving a desired polarized illumination setting by rotating the polarization direction and for putting together the intensity distribution in the pupil plane from appropriately polarized light components. In respect of the prior art, reference is made in purely exemplary fashion to WO 2009/054541 A2. However, in practice, the situation may also arise where at least parts of the illumination light are unpolarized. This is particularly the case in systems in which the light source generates light which is unpolarized from the outset, i.e., for example, in a projection exposure apparatus designed for EUV or else in an illumination device utilizing the i-line (with a wavelength of approximately 365 nm) as illumination light.
Furthermore, during the operation of a projection exposure apparatus there is a need to set specific polarization distributions in the pupil plane and/or in the reticle in the illumination device for optimizing the imaging contrast and also to be able to make a change to the polarization distribution during the operation of the projection exposure apparatus. With regard to the prior art concerning changing the polarization distribution in projection exposure apparatuses designed for the EUV range, merely by way of example reference is made to DE 10 2008 002 749 A1 and US 2008/0192225 A1.
Furthermore, and according to a further aspect of the present disclosure, there may also be a need to at least partly depolarize illumination light for certain illumination settings. Realization of such at least partly depolarized settings may in particular raise problems in situations where the light is already polarized, e.g. due to the use of a polarized light source. Realization of at least partly depolarized settings may in particular be difficult in EUV systems, since conventional depolarizing concepts (such as the use of a so-called Hanle-depolarizer in combination with a light mixing system) are not available in the EUV range due to the non-availability of optically transmissive components.